Using video-intensification fluorescence microscopy, the thyroid hormone, 3,3'5-triiodo-L-thyronine (T3) has been shown to enter cultured fibroblasts by receptor-mediated endocytosis. Studies using GH3 cells and monodansylcadaverine, a potent inhibitor of receptor-mediated endocytosis, showed that cellular uptake as well as incorporation of (I-125)T3 into nuclei was inhibited. These results demonstrate that receptor-mediated endocytic pathway of T3 entry into cells is physiologically significant.